During the welding of pressure vessels, the weldment must be maintained at preheat temperatures until the welding process has been completed and the vessel has been stressed relieved. Minimum preheat temperature is approximately 300.degree. F. but typically temperatures between 400.degree. F. to 450.degree. F. exist. In thick section welding, it is advantageous to inspect the weld during the welding process to detect defects before they are covered by more weld material. In addition, in making difficult welds where defects are quite possible, hot ultrasonic inspection allows repairs to be made without using stress relief time.
Ultrasonic testing of welds at preheat temperatures requires that delay lines be used to protect the transmit transducer and the receive transducer from the high temperatures. The material utilized for the delay lines should have low ultrasonic attenuation and should be readily machineable so as to be conformable to the surface being inspected. Plastics are typically used for delay lines, but at preheat temperatures, plastics exhibit excessive attenuation.
In view of the foregoing, it has become desirable to develop a delay line material which can withstand high temperatures, is machineable, exhibits low ultrasonic attenuation, and is wear resistant.